Modern communication systems evolve towards higher speeds, involving wider and wider communications channels. As an example, GSM systems use 200 KHz channels; 3G systems—5 MHz channels; IEEE802.11b WLANs—20 MHz, 802.11n—80 MHz, UWB WLANs—500 MHz, while the WiGig standard for WLANs at 60 GHz uses 2.16 GHz channels. One of the challenges with the new communications standards is to provide test equipment for characterizing such communications systems. The test equipment needs to provide fidelity of an order of magnitude better than the equipment under test, resulting in use of costly components. This issue is particularly penalizing in test equipment for production lines, where cost is paramount. As the communication products are produced in quantities of tens of millions, and even more (as with the Internet-of-Things trend), the cost of test equipment becomes a critical issue. To take an example, the WiGig modems use internally Analog-to-Digital Converters at 2.64 Gsample/sec, however these converters use relatively low resolution of 6 bits. Test equipment for such equipment would preferably use 9-10 bit converters. However, 10 bit converters at 2.64 Gsample/sec are rare and extremely costly.
It is therefore preferable to have methods and systems for testing communication equipment using low-cost instruments which use lower-cost components. Yet another benefit would be the ability to use legacy test equipment which does not yet support the channel bandwidth of the new equipment under test. Current invention illustrates such methods, taking WiGig tester as an example.
The straightforward solution to testing communications equipment is to down convert a communication signal to a low frequency, filter it to a bandwidth of communications channel, digitize the signal at a rate meeting the Nyquist criterion relative to the channel bandwidth, capture snapshots of data representing communications packets of the system under test, and analyze the collected data. The analysis can be performed in real time (such as using FPGAs) or offline on buffered data (e.g. using general purpose processor)., so as to obtain the performance criteria of interest.
One particular performance criterion for digital communications systems is Error Vector Magnitude (EVM), representing the distortion of the digital modulation “constellation” from its nominal values. The constellations can have differing number of points starting at 2 (for BPSK constellation) and going to 4 points for QPSK, and even larger numbers such as 16, 64, 256 and even higher for QAM (quadrature amplitude keying) modulation. The larger the constellation, the more data is carried by each “symbol” and the higher is the spectral efficiency of the modulation. On the other hand, large modulations require high signal-to-noise ratio at the receiver, and smaller EVM (transmitter distortion) is allowed. As a result, high-efficiency transmitters need to be tested for their EVM, the maximum value of which is typically specified in the standards.